The search for subsurface hydrocarbon deposits typically involves a multifaceted sequence of data acquisition, analysis, and interpretation procedures. The data acquisition phase involves use of an energy source to generate signals that propagate into the earth and reflect from various subsurface geologic structures. The reflected signals are recorded by a multitude of receivers on or near the surface of the earth, or in an overlying body of water. The received signals, which are often referred to as seismic traces, consist of amplitudes of acoustic energy that vary as a function of time, receiver position, and source position and, most importantly, vary as a function of the physical properties of the structures from which the signals reflect. The data analyst uses these traces along with a geophysical model to develop an image of the subsurface geologic structures.
Common Mid Point (CMP) stacking, also sometimes referred to as Common Depth Point or Common Reflection Point (CDP or CRP, respectively), of seismic field data is well known. See, for example, U.S. Pat. No. 3,217,828 to Mendenhall et al., and U.S. Pat. No. 2,732,906 to Mayne, which are incorporated herein by reference as a teaching of the CMP technique. In the CMP technique, redundant data are acquired over the same portion of the subsurface using a plurality of source-receiver offsets. Using what is called the Normal Moveout (NMO) velocity, the redundant seismic traces are stacked to give traces with an improved signal-to-noise ratio.
A comparable procedure is used in seismic imaging in areas with complex structure. Again, a redundant data set is combined using a so-called migration velocity to position seismic events in the proper spatial position.
Implicit in the CMP and the migration process is an assumption that the seismic velocity is isotropic. This assumption was conveniently overlooked for years as the effects were relatively minor, and, given the quality of seismic data available, it was difficult to process the data using an anisotropic velocity model. It is only within the last few years that there has been a sustained effort to account for the effects of anisotropy.
P-wave anisotropy, i.e., a change in the compressional wave velocity with direction of propagation in earth formations due to combined effects of sedimentary layering and the intrinsic anisotropy of the rock. Shales, in particular, could exhibit more than a 20% difference in P-wave velocities parallel to bedding and P-wave velocities perpendicular to bedding. Sandstones and limestones usually show smaller differences in velocity with direction of propagation. Postma (1955) showed that a type of anisotropy called transverse isotropy could be exhibited by seismic waves propagating through a thin layering of isotropic materials.
Determination of anisotropic velocities from surface seismic data is difficult due to the relatively poor data quality and the relatively low frequencies of surface seismic data. Nevertheless, there is prior art on the determination of an anisotropic velocity model for depth imaging of seismic data. See, for example, U.S. Pat. No. 6,864,890 to Meek et al.
Wireline Measurements made in a borehole are generally of higher quality (higher signal-to-noise ratio, commonly called SNR) than surface seismic data. Independent evaluation of the P-Wave velocity components—vertical and horizontal—in an anisotropic formation by conducting conventional wireline logging measurements is not a trivial task. For a vertical borehole (with an axis perpendicular to bedding), the traditional acoustic logging based on analysis of the head wave gives the vertical P-wave velocity component. For a horizontal borehole (with an axis parallel to bedding), the traditional acoustic logging would give the horizontal P-wave velocity component. For an arbitrary borehole inclination, the traditional acoustic logging gives a P-wave velocity that depends upon both the horizontal P-wave velocity component and the vertical P-wave velocity component.
It would be desirable to have a method of determination of seismic velocities as a function of angle of propagation in an earth formation using high quality borehole measurements. The present invention satisfies this need.